1. Field of the Invention
The present invention generally relates to welding equipment and processes. More particularly, this invention relates to a welding apparatus and process for depositing an overlay weld on a boiler tube panel and then straightening the panel for installation in a boiler.
2. Description of the Related Art
Overlay welding generally involves depositing weld material over a surface region in a series of beads with some lateral overlapping, forming a continuous layer of weld material that increases the thickness and strength of the region. Overlay welds are often deposited by shielded metal-arc welding (including gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW)) in the restoration of tubes and large vessels used in industries such as utilities, co-generation refining, petrochemical, pulp and paper, and waste-to-energy. Particular uses of overlay welds include reinforcing and repairing individual boiler tubes, nozzles, and pipe sections of a boiler and the reinforcement and repair of boiler tube panels comprising a number of boiler tubes joined by web or membrane bars. Boiler tubes and tube panels reinforced or repaired with a corrosion-resistant overlay weld offer much longer tube life, reducing costly unscheduled maintenance outages and significantly improving boiler availability.
The reinforcement and repair of boiler tube panels have been performed manually or with the use of an automatic or semiautomatic welding machine. Accuracy and consistency of all parameters are critical during an overlay welding process, especially when the panel must be both supported and manipulated during welding, as is the case with overlay welding operations performed on boiler tube panels prior to installation in a boiler. Further complicating the overlay process is the need to comply with a wide range of specifications, such as minimal weld penetration and deposit thickness, low dilution, complete fusion, homogeneous deposits, and very low heat input.
Distortion has been a particular problem encountered when overlay welding boiler tube panels prior to installation as a result of the large size of tube panels (for example, about fifty feet (about fifteen meters) in length and about six feet (about two meters) in width) and the relatively large heat input of GMAW and GTAW welding process. Distortion in the form of bowing or bending of a panel is particularly likely when welding is performed on only one side of the panel, e.g., the side that will face the interior of the boiler and therefore most likely to be deteriorated from corrosion. As a solution, complex fixtures have been proposed as well as welding techniques that reduce the amount of heating. It would be desirable if an overlay welding apparatus and process were available that were sufficiently uncomplicated for use by operators with limited experience, yet capable of consistently producing undistorted boiler tube panels.